In recent years, studies on lithium secondary batteries with high energy density have been conducted actively, as the miniaturization and lightweight of electronic devices realized and the use of portable electronic equipments generalized. A lithium secondary battery comprises a cathode and an anode that use materials capable of intercalation/deintercalation of lithium ions, and an organic electrolyte or a polymer electrolyte introduced between the cathode and anode. The lithium secondary battery generates energy by an oxidation and reduction reaction when lithium ions intercalate into and deintercalate from the cathode and anode.
Presently, a cathode active material of a lithium secondary battery known in the art includes lithium-manganese composite oxide together with lithium-cobalt composite oxide. Especially, manganese-based active materials such as LiMn2O4 and LiMnO2 are advantageous in that the synthesis is easy with a relatively low production cost and small environmental pollution. However, when the lithium-manganese composite oxide for a lithium secondary battery is repeatedly charged and discharged at 40 to 60° C. or stored for a prolonged period, the life of the battery is shortened due to increased battery resistivity (reduction in the power output) and the capacity reduction.
In order to solve the above problems, Japanese Patent Laid-Open Publication No. Hei 7-153496 teaches to add at least one compound selected from the group consisting of BaO, MgO, and CaO to the lithium-manganese composite oxide, and thereby preventing manganese ions from dissolving into an electrolyte of the battery. However, supposedly, it is difficult to solve the above problems sufficiently. And, the side effects such as initial capacity deterioration by adding a nonconductive compound when constituting a high capacity battery are a concern.